1. Field of the Invention
The present invention relates to methods of predicting the torque and/or drag on a drill string in a directional oil and gas well. More particularly, the present invention relates to improved methods for more accurately predicting and/or analyzing the measured torque and drag of a drill string in such a well to better plan, predict, and control borehole trajectory, to avoid or predict drilling troubles, and to reduce the total cost for the entire well.
2. Description of the Background
As oil and gas exploration becomes more expensive due to more severe environments, there is an increasing urgency to reduce the total drilling, completion, and production cost of a well in order to develop a reservoir more economically. Directional drilling is increasingly being regarded as an effective means to minimize overall development and production cost of an oil field, particularly for the following situations: (1) Drilling multiple directional wells from the same platform or rigsite, particularly in offshore and arctic areas, to reduce rig cost; and (2) Drilling "horizontal" wells to improve production drainage, avoid water coning, and develop very thin reservoirs. While the outlook on directional drilling is very positive, there are many technical problems that need to be resolved in order to further reduce the total cost of a directional well. One such problem concerns the accurate prediction and interpretation of drill string torque and drag data.
Computer models have been used for years to predict drill string torque and drag. The predicted data may be compared to actual or measured torque and drag data, respectively obtained from portable rotary torque meters and weight indicators placed below the kelly and travelling equipment.
Drill string torque and drag data has heretofore been input to a torque drag model, and its findings used for improved well planning design to reduce torque and drag, and for more realistic drill string design and surface equipment selection. On a more limited basis, prior art torque and drag models have been used for rig-site trouble-spotting using diagnostic drilling (tripping) logs by comparing measured and predicted torque and drag to spot potential troubles, and for an aid in casing running and setting. U.S. Pat. No. 4,715,452 discloses a drilling technique intended to reduce the drag and torque loss in the drill string system.
The current drill string torque/drag models, which are widely used in the drilling industry, are each variations of a "soft string" model, i.e. a model that considers the entire length of the drill string sufficiently soft so that the stiffness of the drill string is not taken into consideration. More particularly, the "soft string" torque and drag model: (1) Assumes the drill string to continuously contact the borehole. This implies that effectively the borehole clearance is zero (or rather, no effect of actual borehole clearance is seen); (2) Ignores the presence of shear forces in the drill string in its force equilibrium. Under general conditions, the assumption of zero stiffness does not imply vanishing shears; and (3) For an infinitesimal drill string element, violates moment equilibrium in the lateral direction. For any finite drill string segment, the assumed torque transfer is incorrect.
Since the soft-string model ignores the effects of drill string stiffness, stabilizer placement, and borehole clearance, it generally shows reduced sensitivity to local borehole crookedness and underestimates the torque and drag. Examples of soft string torque and drag models are discussed in the following publications: (1) Johancsik, C. A., Dawson, R. and Friesen, D. B.: "Torque and Drag in Directional Wells--Prediction and Measurement", LADC/SPE conf., SPE paper #11380, New Orleans, 1983, pp. 201-208; (2) Sheppard, M. C., Wick, C. and Burgess, T. M.: "Designing Well Paths to Reduce Drag and Torque", SPE paper #15463, Presented at SPE Conf., October 1986, New Orleans, p. 12; (3) Maidla, E. E. and Wojtanowicz, A. K.: "Field Comparison of 2-D and 3-D Methods for the Borehole Friction Evaluation in Directional Wells", SPE paper #16663, Presented at SPE Conf., September 1987, Dallas, pp. 125-139, Drilling; and (4) Brett, J. F., Beckett, C. A. and Smith, D. L.: "Uses and Limitations of a Drill string Tension and Torque Model to Monitor Hole Conditions", SPE paper #16664, Presented at SPE Conf., September 1987, Dallas, pp. 125-139, Drilling. These references disclose the use of the torque and drag model to plan the directional well path for reduced torque and drag, to estimate the maximum drill string load in order to help in the design of the drill string, and/or to infer borehole quality from the difference between downhole weight on bit (WOB) and surface WOB.
As noted above, each of the softstring models neglects the stiffness of the drill string, and is independent of the clearance between the drill string and the borehole wall. As a result, effects of tight holes and severe local hole crookednesses cannot be easily detected by such a model. The soft-string model thus generally underestimates the torque and drag, or overestimates the friction coefficient. Accordingly, the usefulness of the soft-string model as a rigsite monitor/advisory tool for trouble-spotting is severly limited.
In view of these limitations, some companies have reportedly incorporated a stiffness correction factor to the soft-string model. While this correction factor, when used, will increase the torque and drag for the model to more closely approach the actual measured torque and drag, it does not provide a reliable model for torque and drag predictions to play a major role in well planning, drilling operation (trouble diagnosis and prevention), casing running/setting operations, and completion/cementing operations.
The disadvantages of the prior art are overcome by the present invention, and improved methods and techniques are hereafter disclosed which provide a more reliable and more meaningful torque and drag model which may be used to reliably predict torque and/or drag, and thereby more successfully and economically drill and complete a directional oil or gas well.